From Halopedia, the Halo wiki
In January 2553, the patrol craft UNSC Ariadne experienced complications and had to drop out of slipspace near the Insurrectionist-controlled colony of Venezia. The Monte Cassino was ordered to evacuate the ship, due to the problems the engineers were having fixing the reactor. However, the Monte Cassino was delayed and the UNSC Port Stanley was preparing to conduct the rescue operation, but they failed to do so before the Ariadne's reactor detonated.
Cerny felt so responsible for being late, he tried to make up by searching for survivors by conducting search-and-rescue operations in Venezia's orbit, but the Monte Cassino was fired upon by paranoid insurgent ground forces with a Covenant T-38 "Tyrant" anti-air cannon they had managed to procure. The ship evaded fire and easily destroyed the cannon with missiles. Prior to the Monte Cassino's return trip to Earth, the UNSC Port Stanley transferred ONI operative Mike Spenser and civilian Tom Muir to the ship.
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